EPSC Abstracts
Vol. 7 EPSC2012-413-1 2012
European Planetary Science Congress 2012
c Author(s) 2012
EPSC
European Planetary Science Congress
Stability of methane clathrate hydrates in Titan’s interior:
Experimental constraints at high pressure.
L. Bezacier, G. Tobie, O. Grasset, O. Bollengier, , E. Le Menn and A. Oancea
Université de Nantes, CNRS, Laboratoire de Planétologie et de Géodynamique de Nantes, UMR 6112, Nantes, France
([email protected] / Fax: +33-251125268)
1. Introduction
The origin of Titan’s atmospheric methane has long
been debated. Its continuous destruction by photochemical processes in the upper atmosphere suggests
that it is renewed by internal outgassing in order to
maintain its abundance at the observed value. However, the reservoir location in the interior as well as
the timing and mechanism of outgassing remain unconstrained.
Methane may be stored in the interior in the form
of clathrate hydrates. Clathrate hydrates are nonstoichiometric inclusion compounds with an ice lattice forming molecular cages, in which gases, here
methane, are trapped. Previous studies indicated that
methane hydrate may be stable at high pressure [1, 2],
but their dissociation temperature has never been determined at pressure above 1.5 GPa.
Gravity measurements performed by the Cassini
Radio Science Experiment [3] indicate that Titan’s
interior is only moderately differentiated, suggesting
that some internal regions may have remained undifferentiated and relatively cold during most of Titan’s
evolution. This is also supported by accretion models,
which show that the deepest part of Titan’s interior is
undifferentiated at the end of the accretion stage. In
order to determine the stability of methane in undifferentiated layers of Titan and on the fate of methane
during the differentiation process, we have conducted
high pressure experiments in a Diamond Anvil Cell on
synthesized samples of methane clathrates. We determine the temperature at which methane clathrate dissociates and discuss the implications for the methane
cycle on Titan.
2. Experimental method
In order to study the dissociation curve at temperature and pressure conditions relevant for Titan’s interior, we synthetized methane hydrates in a pressurized vessel and then studied them at high pressure -
high temperature in a Diamond Anvil Cell. Raman
spectroscopy is used as a tool to discriminate methane
trapped into clathrate structures from solid methane
mixed with solid water ice and their respective structures at high pressure (between 1.5 and 5 GPa). We
also conducted a high pressure study of pure methane
to establish the Raman spectra (Figure 1a).
Figure 1: (a) Raman spectra of methane hydrate (blue)
and solid methane (black) at high pressure (3.9 GPa)
(b) Dissociation curve of methane hydrates compared
to the melting curve of water ices. Black diamonds are
experimental data obtained by [2] up to 1.5 GPa and
the melting curve of methane is plotted in orange.
3. Results
Acknowledgements
As illustrated on Figure 1a, pure solid methane spectra are different at high frequencies (2900-3300 cm-1 )
from those of the methane hydrates. The primary peak
is shifted by about 4 cm-1 in the MH spectra relative to
the solid methane spectra. The solid methane spectra also exhibit a secondary peak, which is missing
in the MH spectra. These spectra differences permit
us to distinguish methane hydrate from a mixture of
solid methane and ice [4, 5] and to ensure that we
really observe MH dissociation when increasing the
temperature. Raman spectra from methane hydrates
MH-II and MH-III have also been obtained and they
have been used to confirm the phase transition occuring at 2 GPa. As displayed on Figure 1b, we have a
good agreement with published data at pressure below
1.5 GPa. In this pressure range, the dissociation of
MH is above the melting point of water ice. At higher
pressure (> 1.5 GPa), our experiments show that MH
destabilizes at temperature very close to those of water
ice melting.
This has important consequences for the evolution
of methane in Titan’s interior. As long as the internal temperature is below the melting point of water,
methane should remain trapped in the deep interior
in the form of clathrate hydrate. If the interior differentiates slowly without ice melting, large quantities of methane may still be trapped in the outer part
of the rock-rich core, and methane clathrate should
slowly migrate from the deep interior to the outer ice
shell. On the other hand, if significant ice melting occurs during the differentiation process, all the methane
should be extracted very rapidly from the core. A large
fraction of the released methane should be retrapped
in the form of clathrates as they are stable in the ocean
conditions (P < 1 GPa), and could form a thick layer
of methane clathrate above the ocean (e.g. [6]).
The research leading to these results has received
funding from the European Research Council under the European Community’s Seventh Framework
Programme (FP7/2007-2013 Grant Agreement no.
259285).
4. Conclusion
We have conducted the first high-pressure hightemperature studies on the destabilization of methane
hydrates at pressure above 1.5 GPa and we show that
their destabilization occurs close to the melting point
of ice.
These results are also applicable to water-rich exoplanets, where methane hydrates may remain stable at
high pressure and affect the composition of their atmosphere.
References
[1] Choukroun, M., Grasset, O., Tobie, G.,
Sotin, C.: Icarus, pp 581-593, 2010.
[2] Dyadin, Y.A., Aladko, E.Y., Larionov, E.G.:
Mendel. Comm., 7, pp. 34-35, 1997.
[3] Iess, L., Rappaport, N. J., Jacobson, R. A.,
Racioppa, P., Stevenson, D. J., Tortora, P.,
Armstrong, J. W., and Asmar, S. W.: Science
327, pp 1367-1369, 2010.
[4] Ohtani, T., Ohno, Y., Sasaki, S., Kume, T.
and H. Shimizu: Journal of Physics: Conference Series, 215,2010.
[5] Shimizu, H., Kumazaki, T., Kume, T. and
Sasaki, S.: Journal of Physical Chemistry, B,
106, pp 30-33, 2002.
[6] Tobie, G., Lunine, J.I., Sotin, C.: Nature,
440, pp 61-64, 2006.